This invention relates to a time division multiplex system and method for the transmission of a plurality of items of data composed of start-stop signals with a steady transmission speed and a fixed signal frame, and of a plurality of items of data composed of start-stop signals of arbitrary transmission speed and arbitrary signal frames.
In future data networks the majority of subscribers will employ start-stop end devices of low transmission speeds. These include, for example, teleprinter devices with a transmission speed of 50 Bd. and a signal frame with 7.5 elements or bits, and data terminal devices with a transmission speed of between 50 and 200 Bd and signal frames in the region of 7.5 to 12 elements. Technical development has resulted in time division multiplex systems currently being more favored in some cases than the frequency multiplex systems which were previously mainly employed. Therefore, for the connection of the subscriber terminal devices to the exchanges one seeks multiplex systems which in the speech channel can offer a sufficient number of channels for the teleprinter terminal devices and channels for the data terminal devices.
In order to group channels for start-stop signals with a steady transmission speed and a fixed signal frame, time division multiplex systems with a signal-frame-controlled mode of operation are known, which for each element produced by the terminal device require one bit in the transmission channel. These systems make good use of the capacity of the multiplex transmission link. In such systems only data with known, constantly equal signal frames are transmitted in one and the same channel. This means that the actual information is embedded in a time frame composed of a start element and a stop element of at least 1.5 times the size of the signal frame. Signal-frame-controlled systems of this type are not suitable for channels which must permit arbitrary transmission speeds and signal frames within certain limits. However, speed-transparent multiplex systems are known (U.S. Pat. No. 3,535,450, German AS 2 005 836) with the aid of which channels of this type can also be grouped. As used herein, the term "speed-transparent" refers to a communications transmission channel capable of permitting the transmission of arythmnical messages at all data signalling rates less than a maximum rate for the channel. These systems operate with multiple scanning of each date element and coding of the time state of the polarity change. For each element of the end device they require a plurality of bits in the transmission channel; the number of these bits is dependent upon the permissible engagement error. The requirement for transmission capacity in these systems is thus considerably higher than in the signal-frame-controlled systems. If such a system is used to group the data channels and the teleprinter channels, the total number of channels which may be accommodated in the speech channel is insufficient.
It is, therefore, an object of the invention to provide a time division multiplex system which permits an optimum exploitation of the capacity of the multiplex transmission line.